Introduce a role_owner group into the PG permission system

Introduce a role_owner group into the PG permission system

This new role is used to assign permissions.  (The permissions granted
by membership in other roles.)  This prevents users from losing their
permissions when the creating role is dropped, as when an
administrator leaves.  It pushes the chicken-and-egg problem back to
the administrator level.  This is then handled by saying in the docs:
"Don't use a role that will go away when creating administrators."